Variable capacitors are used with inductor coils in tuning circuits of radios, television sets, and a number of other devices that must isolate electromagnetic radiation of selected frequencies. In the circuit below, a variable capacitor is connected to a simple antenna transformer circuit.

Click and drag on the slider bar in order to adjust the capacitor to tune the "radio" receiver to a selected frequency. Isolated radio-frequency wavelengths can be seen on the oscilloscope screen.

Transmitted radio waves cause an induced current to flow in the antenna through the primary inductor coil of the transformer to ground. A secondary current in the opposite direction is induced in the secondary inductor coil of the transformer sending an electron flow to the capacitor. The induced current flow in the secondary coil, and to the capacitor, induce counter electromotive forces called reactance. The variable capacitor is used to equalize the inductive and capacitive reactance.

The condition in which the reactances are equalized is called resonance. The particular frequency that is isolated by the equalized reactance is called the resonant frequency. This radio circuit therefore, is tuned by adjusting the capacitance of the variable capacitor to equalize the inductive and capacitive reactance for the desired resonant frequency, or in other words, to tune in the desired radio frequency.